1. Field of the Invention
The present invention relates generally to lighting systems and more particularly to a light projector with improved means for controlling the focus, size and direction of a projected beam of light.
2. Discussion of the Prior Art
Conventional stage lighting systems and spot lights have used triplet lenses to focus and vary or "zoom" the size of a projected beam of light. U.S. Pat. No. 3,128,329 uses a well-known Cooke triplet in combination with a negative doublet fourth lens to magnify and focus a light beam. The triplet lenses are mounted fixedly with respect to each other and movably along the optical axis with respect to the fourth lens to adjust the size of the beam. The triplet and fourth lens are movable together relative to the back-focal object plane of a transparency to be imaged to focus the image at the selected size.
U.S. Pat. No. 3,649,113 discloses a projector in which an objective lens and a unitary zoom lens are manually adjusted along their axis to focus an image at a distance after which the zoom lens position may be adjusted to control the image size.
U.S. Pat. No. 4,063,800 describes a projector zoom lens formed from a triplet or three groups of lenses. The outer two lenses are fixed relative to the back-focal object (plane) while the middle lens is relatively movable to vary the size of the projected image.
U.S. Pat. No. 4,118,108 describes a projector zoom lens formed from a triplet including a relatively movable middle lens to vary the beam size, and a movable front lens to focus the beam.
U.S. Pat. No. 4,462,067 describes a spotlight projector including an axially movable light source and a fixedly mounted, adjustable diameter iris to control the size of a projected beam, an axially movable rear lens to focus the beam, and a semi-permanently mounted front lens. Various axial locations are provided with means for semipermanently mounting different types of front lenses to convert the spotlight to shorter or longer "throw" lengths.
U.S. Pat. No. 4,519,020 discloses a variable magnification stage light including two lenses and a light source which are all manually movable along their optical axis to focus and magnify a beam of light.
U.S. Pat. No. 4,627,720 describes a projection in which a zoom lens is axially movable by a lens drive means to adjust the projected image size. A control means responsive to the magnification adjustment of the zoom lens drive controls the focal length.
Lighting systems which move two lenses relative to each other to focus and magnify a light beam must constrain the back-focal object plane and lenses spacings to certain relationships, depending upon the specific characteristics of each lens. Conventional systems in which multiple lens positions are coordinated by mechanical linkages permit only predetermined spacial relationships which would not enable the lenses to focus from different back focal plane distances. Also, such linkages are typically not dimensioned correctly for different types of lenses requiring different axial relationships to focus at any object and/or image distance.
There remains therefore a need for a light projector which can be adjusted simply to use a general range of lens triplets to focus from various back-focal distances to project images at selected sizes at selected front-focal distances.